Question

How to factor: a^2+11a+18

Answer 1

$a^2+11a+18\\\\-----------------------------\\\\11=9+2\\18=9\cdot2\\\\-----------------------------\\\\a^2+11a+18=(a+9)(a+2)$

Answer 2

$a^2+11a+18=a^2+2a+9a+18=a(a+2)+9(a+2)=\\\\=(a+2)(a+9)$